This invention relates to a composite magnetic body and a method of manufacturing the composite magnetic body. The composite magnetic body is usable as an electromagnetic interference suppressor for use in an electronic apparatus to prevent leakage of an extraneous radio wave to the outside, interference between internal circuits, and influence of an external radio wave such as malfunction.
In recent years, it is a matter of great concern to cope with a problem as to an electromagnetic environment of an electronic apparatus. In the background, there are various requirements for an electronic equipment such as a digital electronic apparatus to increase a signal processing speed of a circuit, to use a higher frequency, to provide improved functions, to reduce the size and the thickness of a product design, and so on. To meet these requirements, the circuit is increased in density so that active elements and passive elements, such as semiconductor elements, which emit inductive noise are mixedly used.
This results in an increase in line coupling due to capacitive coupling and electromagnetic coupling and in occurrence of interference due to radiation noise to often inhibit normal operation of the apparatus. Sometimes, such noise may have an influence upon external apparatuses.
As an anti-noise countermeasure in the electronic apparatus improved in processing speed, function, and density, particularly, an anti-noise countermeasure in a submicrometer wave band, it is proposed to mount a low pass filter and to use shielding. In the anti-noise countermeasure by the use of an additional component, a mounting space is required so that consideration must be made at a design stage in order to reduce the size and the thickness. Therefore, an emergent demand can not be met.
In view of the above-mentioned problems, proposal is made of a composite magnetic sheet suppressing the above-mentioned electromagnetic interference as the anti-noise countermeasure in the submicrometer wave band. For example, proposal is made of composite magnetic sheets capable of suppressing electromagnetic interference, as described in Japanese Unexamined Patent Publication (JP-A) No. H07-212079 and the specification of Japanese Unexamined Patent Publication (JP-A) No. H09-35927.
As a radio absorber covering the submicrometer wave band and utilizing a high real part permeability μ′ and a widespread imaginary part permeability μ″, these composite magnetic sheets can suppress transmission of the radiation noise and secondary electromagnetic coupling. Therefore, it is possible to lighten the load involved in the anti-noise countermeasure and product development and to remove the bar to reduction in size and increase in density of the circuit.
In the market, there arises a demand for an anti-noise component which can be conveniently used, like the above-mentioned composite magnetic body, in a frequency band allowing the use of the component as the countermeasure, i.e., around a low frequency (several tens to a hundred MHz) within the submicrometer wave band and which can be mounted to an existing apparatus. Furthermore, with the progress of miniaturization of the electronic apparatus, the composite magnetic sheet adapted to the above-mentioned submicrometer wave band is also required to be further reduced in thickness.
The above-mentioned composite magnetic sheet used in the submicrometer wave band becomes insufficient in permeability as the frequency is lowered. Therefore, reduction in thickness is limited and the effect of suppressing electromagnetic interference is decreased, as will be understood from the frequency at which μ″ rises up. In a small-sized electronic apparatus such as a mobile communication apparatus, consideration will be made of another countermeasure. However, ferrite can not be mounted because of the possibility of fracture and the problem of the location. Rubber ferrite, which will hardly be broken, can not sufficiently suppress the noise although a mounting space is assured.
Thus, in lower and higher frequency ranges than several tens to a hundred MHz, use can be made of those components such as a coil and a filter. However, in the above-mentioned frequency band, any convenient anti-noise component is not found at present. Even if there is such adaptable component, much cost and manhour are required in design modification of a substrate and the like.
In order to solve the above-mentioned problem, there arises a demand for a composite magnetic sheet for suppressing electromagnetic interference, which is of the type adapted to a lower frequency range (several tens to a hundred MHz) and exhibiting an electromagnetic interference suppressing effect even if the thickness is reduced.
To meet the above-mentioned demand, it is essentially required to improve the permeabilities μ′ and μ″ and to lower a magnetic resonance frequency of μ′. In response to these requirements, for example, the specification of Japanese Unexamined Patent Publication (JP-A) No. 2000-4097 proposes a composite magnetic sheet which is improved in electromagnetic interference suppressing effect in a FM band (around several tens to a hundred MHz) and adapted to reduction in thickness. By the use of the technique described in the above-mentioned application, the improvement in electromagnetic interference suppressing effect and the reduction in thickness have been achieved.
Herein, description will be made of one example of a method of forming the above-mentioned composite magnetic body. In case of the above-mentioned composite magnetic sheet adapted to the submicrometer wave band, use may be made of a wet method and a dry method. Specifically, in the wet method, flat soft magnetic powder, a binder, and a solvent for dissolving the binder are mixed and agitated to obtain a slurry-like mixture which is formed into a film by the use of the doctor blade technique. In the dry method, flat soft magnetic powder and a binder are kneaded by a kneader to obtain a mixture which is formed by rolls.
In a method of producing a composite magnetic body described in the above-mentioned Japanese Unexamined Patent Publication (JP-A) No. 2000-4097, flat soft magnetic powder free from stress strain is used and formed into a sheet by the wet method so as not to give the stress strain to the powder during the forming step. Furthermore, the sheet is pressed by the use of a press in a direction perpendicular to a sheet plane to realize high density. Thus, the electromagnetic interference suppressing effect in the FM band is improved and the reduction in thickness is achieved.
As described above, the conventional method of producing the composite magnetic body comprising the flat soft magnetic powder and the binder is classified into the wet method using the solvent and the dry method using no solvent.
The method of producing the composite magnetic body described in Japanese Unexamined Patent Publication (JP-A) No. 20004097 adopts the above-mentioned wet method. However, a sufficient value of μ′ can not be obtained by film formation alone and, therefore, pressing by the use of the press is performed to achieve high permeability. Therefore, it is difficult to reduce the cost in case where the production amount is increased. In order to perform mass-production by the use of this method, a large-scale plant investment is required.
In the above-mentioned dry method, various external forces are often applied to the flat soft magnetic powder during the machining process. In this event, mechanical stress strain is produced again in the flat soft magnetic powder free from stress strain so that desired magnetic characteristics can not be obtained. Practically, decrease in magnetic resonance frequency fr of the real part permeability μ′ and decrease in the values of μ′ and μ″ have been confirmed.
From the abovementioned reasons, a number of problems exist when the composite magnetic body adapted to the band between several tens and a hundred MHz is produced with the above-mentioned method.